Kelp needed as Port Phillip Bay seafloor cleared by march of urchins

It wasn’t until Paul Carnell lined up the old satellite photos of Port Phillip Bay with fresh images that he saw it.

"Whoa", Dr Carnell thought, as he looked over the old images. There were thick forests of kelp everywhere, supporting a rich ecosystem. In the new images, those forests were now barren, a desert on the seafloor.

In a stroke, they proved the claims of locals: starving sea urchins were destroying huge swathes of the bay.

“In some of the images in the 1970s and '80s, you can start to see urchin barrens [areas of the seafloor denuded of all life] on parts of the reef, particularly deeper, where people would not have noticed,” says Dr Carnell, who is a fellow at Deakin University.

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“But then through the 2000s you see the urchins coming from the deep and just clearing everything. You go from lots of kelp to basically nothing.”

This image shows the huge damage done by the sea urchins.Credit:Nearmap.com.au

Sea urchins are native to the bay, and normally live in balance with its other inhabitants. They feed off drifting seaweed. Their orange innards are considered a delicacy in some Asian countries – locally, one kilogram of sea urchin roe sells for $100.

“They are spectacularly colourful, and they’ve got big spikes for warding off predators. They are really quite cool things,” says Ray Lewis, a local who regularly dives at Ricketts Point.

Sea urchins can be quite beautiful – when they are not destroying everything in sight.Credit:Raymond Lewis / Supplied

But in about 2007, Mr Lewis and a group of other locals grew concerned. Around Ricketts Point, and at Point Cook and Williamstown, there suddenly seemed to be a lot more urchins and a lot less kelp.

“So we started making heavy weather about it,” says Mr Lewis. The group lobbied the government to formally study the problem, but did not make much headway. He almost gave up. “I thought ‘bugger it, I’m getting old, I’ve got lots of other projects’,” says Mr Lewis.

Eventually, the Department of Environment, Land, Water and Planning asked Dr Carnell to do a study. The results were a shock.

The creatures normally mass about two to three per square metre; Dr Carnell found patches of the bay with up to 20 urchins chomping away at kelp.

The denunded area, highlighted in this illustration by Dr Carnell.Credit:Nearmap.com.au

Using archival data and the satellite images, Dr Carnell was able to develop a timeline for the urchin explosion. He believes it may have been driven by reduced rainfall during southern Australia's Millennium drought.

Rainfall washes nutrients into the sea, encouraging kelp growth. Without it, the kelp dies off, leaving the urchins hungry. The starving creatures then have to move for their next meal, heading into the bay's kelp forests and devouring them.

“They just plough down the kelp,” says Dr Carnell. “In 60 years, there has never been this much urchin barren in the bay.”

Last Friday Dr Carnell steered a boat out into the choppy waters around Point Cook. On board were a small team from Parks Victoria – which was funding the trip – and Deakin University, plus a toolshed-worth of hammers.